Documentation creation date

[jhlegarreta]

It was identified in DIPY's documentation configuration file that using datetime.now() to generate the current date for the documentation may lead to a regression if the documentation of a previous release is generated and the date used when rendering that documentation (e.g. the Copyright year).

Using the SOURCE_DATE_EPOCH environment variable for this purpose seems to solve the issue.

Cross reference to explanation in the related DIPY PR: dipy/dipy#3423 (comment).

Applies to all NiPreps.